When I talk to a transmog NPC with my gear equipped the window shows my gear flashing randomly between what it’s transmogged to and saying I have nothing equipped, and the game starts to lag really bad.
I tried taking all my armor off and talking to the NPC, then putting the armor on in the window, and that worked, but it’s really inconvenient to have to do in order to change my appearance.
edit*
It’s not random, it first shows my legs, chest, cloak, gloves, and weapons, and everything else as unequipped. Then it shows everything else, with the legs, chest, and weapons unequipped, it shows the gloves and cloak as their item not their transmog. Then it shows everything as unequipped.
This just cycles, with the game lagging and never giving my any ability to give input to the system.
last edit*
The shoulders, tabard, and belt can be equipped when talking the the transmog NPC and it works, adding any other piece breaks it.